The present invention relates to a process for the catalytic incineration of residual gases containing a low content of at least one sulfur compound selected from COS and CS.sub.2, and mercaptans, and possibly at least one member of the group formed by H.sub.2 S, SO.sub.2, vapor and/or vesicular sulfur. It also relates more especially to a process for the catalytic incineration of residual gases issuing from CLAUS units in the preparation of sulfur.
Various industrial operations give rise to residual gases containing a low content of H.sub.2 S and/or SO.sub.2 as well as other sulfur compounds such as COS and/or CS.sub.2, and/or mercaptans, and possibly vapor and/or vesicular sulfur. Especially residual gases issuing from a unit sulfur manufacture by the gentle oxidation process with hydrogen sulfide, known as the CLAUS process, contain about 0.5 to 2% by volume of sulfur compounds, a high proportion of which consists of H.sub.2 S, the remainder being formed of SO.sub.2, COS, CS.sub.2 and vapor and/or vesicular sulfur.
To comply with the most stringent standards imposed by legislation covering atmospheric pollution, these residual gases are subjected to thermal incineration before being rejected into the atmosphere. The purpose of this incineration is to transform into SO.sub.2 the sulfur compounds such as H.sub.2 S, COS, CS.sub.2 and vapor and/or vesicular sulfur, since rejection into the atmosphere of SO.sub.2 is not subject to as strict limitations as those imposed on the rejection of other sulfur compounds in the form of gaseous sulfur, especially H.sub.2 S, COS and CS.sub.2.
In this thermal incineration step, the residual gas to be incinerated is placed in contact with a suitable quantity of a gas containing oxygen, usually air, and at a temperature sufficient to provoke oxidation into SO.sub.2 of vapor and/or vesicular sulfur as well as gaseous sulfides, namely H.sub.2 S, COS and CS.sub.2, which are present in the residual gas. The calories necessary to maintain the temperature in the incineration zone at an appropriate level are produced by combustion of a suitable combustible gas injected in the incineration zone at the same time as the air and the residual gas to be incinerated. During operation of this thermal incineration high energy is consumed since it is necessary to operate at high temperature most often in the order of 700.degree. to 800.degree. C., to transform into SO.sub.2 all the oxidable sulfur compounds present in the residual gas subjected to the incineration.
With the aim of reducing energy consumption needed by the thermal incineration, it was proposed to incinerate catalytically the residual gases containing H.sub.2 S and possibly other oxidable sulfur compounds into SO.sub.2 by bringing the residual gas in contact with a gas containing oxygen in the presence of an appropriate catalyst and at a temperature in the range of 200.degree. to about 550.degree. C. The catalyst used is most often formed by an alumina support or matrix impregnated with a metal compound, such as for example copper, zinc, iron, cobalt, nickel, manganese, possibly associated to a metal or metalloid compound of groups IV to VI of the Periodic Table. A process of this type is described in F.P.A. No. 78.00205 of Jan. 5, 1978 (Pub. No. 2 376 686) the said process using a catalyst comprising an alumina support or matrix impregnated with copper and bismuth compounds. In these catalytic incineration processes as in the thermal incineration but at a lower temperature, is carried out a SO.sub.2 oxidation of hydrogen sulfide and possibly other oxidable sulfur compounds present in the residual gas, after, in certain cases, the said residual gas having been subjected to a hydrogenation treatment to transform all the sulfur compounds contained in said gas into H.sub.2 S which compound is easily oxidable in SO.sub.2.
Although these catalytic incineration processes permit operation with an energy consumption clearly lower than the thermal incineration, they are not entirely satisfactory since the oxidation catalysts on the alumina support or matrix used for the transformation into SO.sub.2 of the hydrogen sulfide and possibly other sulfur compounds such as COS, CS.sub.2, vapor and/or vesicular sulfur rapidly deactivate, thus after a life-span of the said catalysts which is relatively short on the industrial level, leading to a sharp decrease in the rate of conversion into SO.sub.2 of the sulfur compounds contained in the treated residual gas. This results in risks of rejection into the atmosphere of highly polluting sulfur compounds, such as for example H.sub.2 S, in concentrations higher than those stipulated by the current standards covering atmospheric pollution. Thus for a catalyst comprising an alumina support impregnated with an iron compound, the conversion rate of SO.sub.2 into hydrogen sulfide, which was originally equal to 100% drops to 75% after 4000 hours use, a relatively short life-span on the industrial scale.
The present invention relates to a process analogous to the hereinabove processes, with a prior hydrogenation, for the catalyst incineration of residual gases containing at least one sulfur compound selected from among COS, CS.sub.2, mercaptans, and possibly at least one member of a group formed by H.sub.2 S, SO.sub.2, vapor and/or vesicular sulfur which however allows the drawbacks of the said processes to be overcome by providing means of achieving oxidation of H.sub.2 S into SO.sub.2 with a conversion rate maintained throughout at a steady level substantially equal to 100%.
The process according to the invention for the catalytic incineration of residual gases containing a low content of at least one sulfur compound selected from among COS, CS.sub.2, mercaptans and possibly at least one member of the group formed by H.sub.2 S, SO.sub.2, vapor and/or vesicular sulfur, is of the type comprising a first stage in which the residual gas is subjected to a hydrogenation treatment with the aim of obtaining sulfur compounds it contains in the sole form of H.sub.2 S, and a second stage in which the gaseous effluent of the hydrogenation treatment is placed in contact with an appropriate quantity of a gas containing oxygen, at a temperature comprised between about 150.degree. C. and 570.degree. C. and in the presence of oxidation catalyst of H.sub.2 S into SO.sub.2, wherein the oxidation catalyst is formed of a porous support or matrix having a surface area at least equal to 5 m.sup.2 /g and constituted of 50 to 100% by weight of at least one compound selected from among silica, zeolites, titanium oxides (expressed as TiO.sub. 2), and zirconium oxides (expressed as ZrO.sub.2) and from 50 to 0% by weight of alumina, to which are associated one or several compounds of metals belonging to the group constituted by copper, silver, zinc, cadmium, yttrium, lanthanides, vanadium, chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, rhodium, iridium, palladium, platinum, tin, and bismuth, the oxidation catalyst support or matrix always containing a titanium oxide and/or a zirconium oxide when a copper compound and a bismuth compound are jointly associated to the support or matrix as compounds of the metals of the said group.
During the first stage of the following process according to the invention, sulfur compounds such as SO.sub.2, CS.sub.2, COS, mercaptans, as well as vapor and/or vesicular sulfur are transformed into H.sub.2 S in the presence of hydrogen. Hydrogenation reaction in the absence of a catalyst would make it necessary to work at high temperatures, and because of this it is preferable to carry out this hydrogenation in the presence of a catalyst, thus enabling operating at lower temperatures. More precisely, the catalytic hydrogenation treatment is carried out at temperatures from about 140.degree. to about 550.degree. C. and preferably between about 200.degree. C. and 400.degree. C. The hydrogen required for the hydrogenation reaction may already be contained in the residual gas or may be formed in situ in the hydrogenation zone, for example by reaction of CO on H.sub.2 O when the residual gas contains these two reactives, or otherwise may be added to the residual gas from an external hydrogen source feed. The quantity of hydrogen to be used must be sufficient to obtain a substantially complete transformation into H.sub. 2 S of the hydrogenable sulfur compounds or products, such as SO.sub.2, COS, CS.sub.2, mercaptans, vapor and/or vesicular sulfur, contained in the residual gas subjected to the hydrogenation treatment. In practice, the quantity of hydrogen used may range from 1 to 6 times the stochiometric quantity required to transform into H.sub.2 S the hydrogenable sulfur products present in the residual gas.
The catalysts which can be used for the hydrogenation treatment are those which contain metals of the Va, VIa and VIII groups of the Periodic Table, for example, metal compounds such as cobalt, molybdenum, chromium, vanadium, thorium, nickel, tungsten, uranium, the said compounds being placed (or not) on a support or matrix of the silica, alumina, silica/alumina type. Particularly efficient for the hydrogenation treatment are hydrodesulfuration catalysts based on cobalt and molybdenum oxide supported on alumina. For this hydrogenation treatment, contact time between the gaseous reaction medium and the catalyst can vary fairly widely. They are preferably located between 0.5 and 5 seconds and more especially between 1 and 3 seconds.
During the second stage of the process according to the invention, the gaseous effluent issuing from the hydrogenation treatment, i.e. from the first stage of the process, is contacted with an appropriate quantity of a gas containing molecular oxygen, at a temperature comprised between 150.degree. C. and 570.degree. C., the said temperature being more particularly located between about 200.degree. and 570.degree. C. and preferably between about 250.degree. and 570.degree. C., in the presence of a specific oxidation catalyst, which ensures a complete conversion into SO.sub.2 of H.sub.2 S forming the sole sulfur product in the said effluent, and the gaseous phase resulting from this oxidation, which contains SO.sub.2 as the sole sulfur compound, is thereafter rejected into the atmosphere.
The gas containing the molecular oxygen used in this second stage of the process according to the invention is generally air, although it is possible to use pure oxygen, oxygen-enriched air, or even other mixtures, in varied proportions, of oxygen and an inert gas other than nitrogen. The gas containing the oxygen is used in such a quantity that there is an oxygen quantity at least equal to and preferably higher than the stoechiometric quantity necessary for oxidation into SO.sub.2 of the total H.sub.2 S contained in the gaseous effluent issuing from the first stage. Advantageously, the quantity of gas containing oxygen is such that the molecular quantity of oxygen represents 1.5 to 10 times the molecular quantity of H.sub.2 S to be oxidized into SO.sub.2. The gaseous effluent issuing from the hydrogenation treatment and the gas containing oxygen can be brought separately into contact with the oxidation catalyst. However, with the purpose of obtaining a very homogenous reaction medium, it is preferable to mix firstly the said effluent with the gas containing oxygen and to contact the mixture thus formed with the oxidation catalyst.
For this oxidation step, contact times of the gaseous reaction medium with the oxidation catalyst may range advantageously from 0.1 to 6 seconds and preferably be comprised between 0.5 and 3 seconds.
The duration of the contact times for both the hydrogenation step and the oxidation step are expressed under the classic pressure and temperature conditions.
The porous support or matrix of the specific oxidation catalyst used in the second step of the process according to the invention is constituted, as hereinabove mentioned, by 50 to 100% by weight of at least one product selected from among titanium oxides, expressed as TiO.sub.2, zirconium oxides, expressed as ZrO.sub.2, silica, and zeolites, especially the faujasite, mordenite and ferrierite type, and from 50 to 0% by weight alumina.
By the expression "titanium oxide or zirconium oxide" is meant in the present disclosure a oxidized titanium or zirconium compound which according to the methods of preparation used for its obtention, can comprise titanium or zirconium dioxide or otherwise oxyhydrate type compounds.
The porous support or matrix of the oxidation catalyst presents a surface area, determined in accordance with BET method, of at least 5 m.sup.2 /g, especially 20 to 800 m.sup.2 /g and preferably 50 to 600 m.sup.2 /g.
The metal compounds, that are associated to the porous support or matrix defined hereinabove, are in particular compounds, especially oxides, mineral or organic acid salts, such as sulfates, nitrates, phosphates, acetates of metals taken from group A constituted by iron, cobalt, nickel, manganese, copper, zinc and cadmium, or from group B constituted by vanadium, bismuth, chromium, molybdenum and tungsten. One or several metal compounds from group A and one or several metal compounds from group B can also be simultaneously associated to the support or matrix.
One or several metal compounds taken from group C constituted by copper, silver, molybdenum, tungsten, iron and bismuth and one or several metal compounds taken from group D constituted by lanthanides, chromium, cobalt, nickel, vanadium, tin, rhodium, palladium iridium and platinum can also be simultaneously associated to the support or matrix.
The overall quantity of the metal compound(s) which are associated to the support can represent, expressed by weight of metal 0.005 to 25% by weight of the calcinated catayst and said quantity is preferably between 0.01 and 20% of this weight.
When one or several metal compounds of group A and one or several metal compounds of group B are simultaneously associated to the support or matrix, the total number of atoms of the metal(s) of group A/total number of atoms of the metal(s) of group B ratio is comprised between 0.1 and 10.
Furthermore, when one or several metal commpounds of group C and one or several metal compounds of group D are simultaneously associated to the support or matrix, the total number of atoms of the metal(s) of group C/total number of atoms of the non-precious metals of group D ratio is comprised between 0.1 and 10 whereas the total number of atoms of the metal(s) of group C/total number of atoms of the precious metal(s) of group D ratio is comprised between 20 and 1000, and preferably between 50 and 400.
Most suitable oxidation catalysts according to the invention comprise a porous support or matrix such as hereinabove defined, and particularly silica, a titanium oxide of TiO.sub.2 type, titanium oxyhydrate, zirconium oxide of ZrO.sub.2 type or zirconium oxyhydrate matrix presenting a specific surface of at least 5 m.sup.2 /g, particularly 20 to 800 m.sup.2 /g and preferably 50 to 600 m.sup.2 /g, on which is placed an iron compound, and especially iron sulfate, in a quantity such that the weight of iron presents 0.1 to 15%, and preferably 0.5 to 10% by weight of the calcinated catalyst, the iron compound being possibly associated to a precious metal of group D, especially palladium, in a number of iron atoms/number of precious metal atoms ratio comprised between 20 and 1000 and preferably 50 and 400.
The preparation of the oxidation catalyst and that of the support of the said catalyst can be carried out using various methods known per se. For example a silica, titanium oxide or zirconium oxide support or matrix may be obtained by precipitation of a hydrogel of silica, titanium oxyhydrate or zirconium oxyhydrate from respectively sodium silicate, a titanium salt or a zirconium salt, thereafter forming the hydrogel into pellets or beads, and then drying and calcinating the pellets or beads thus obtained. A SiO.sub.2, TiO.sub.2 or ZrO.sub.2 support or matrix may otherwise be obtained by hydrolysis of respectively SiCl.sub.4, TiCl.sub.4, ZrCl.sub.4, pelletizing of SiO.sub.2, TiO.sub.2 or ZrO.sub.2 formed, and drying then calcinating the pellets.
A mixed support or matrix, containing for example a titanium or zirconium oxide and/or SiO.sub.2 jointly with alumina can be produced by mixing the selected oxides or again by a coprecipitation of oxides from titanium or zirconium salts and/or sodium silicate and aluminium salts. The catalyst can be obtained for example by impregnation of a selected support with one or several of the desired metal compounds, then drying of the impregnated support and calcinating the dried product at a temperature comprised especially between 350.degree. C. and 600.degree. C. The association of the afore-mentioned metal compound(s) with the support or matrix can also be achieved by coprecipitation techniques or otherwise by mixing the constituents of the catalyst in the form of oxides.
The process according to the invention is particularly adapted to the incineration of residual gases issuing from installations for manufacturing sulfur according to the CLAUS process, the said residual gases possibly having been furthermore subjected before incineration to a complementary purification treatment by any method known per se in order to reduce further their sulfur compounds content.
In the present description and associated claims, the expression "a low content" relating to the total content of sulfur compounds contained in the residual gas, denotes a content lower than 5% by volume.